Polished polyimide substrate

ABSTRACT

Polymer substrates, in particular polyimide substrates, and polymer laminates for optical applications are described. Polyimide substrates are polished to an average surface roughness of about 0.25μ inch, and single-layer or multi-layer waveguide structures are deposited on the polished polyimide substrates. Laminates including polymer or a hybrid organic/inorganic waveguiding film can be deposited on a polished polyimide substrate. The laminate can also include piezoelectric and metallic layers. Micromachined optical devices, such as cantilevered waveguide are fabricated by laser ablation using a combination of IR and UV lasers. A fiber-to-waveguide coupler with a laser-machined groove for holding the fiber is also disclosed. Holes are drilled in the polyimide wafer using excimer or tripled YAG laser. The holes are metallized to provide mircovias. Metallized polyimide wafers are stacked and attached using flip chip bonding technology to provide three-dimensional high density interconnects for microelectronic chip packaging.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of International ApplicationNumber PCT/US/2004/013493, filed on May 3, 2004, which designated theUnited States (US) and claims priority to U.S. application Ser. No.10/429,080 filed May 2, 2003. The entire disclosure of InternationalApplication Number PCT/US/2004/013493, as initially filed on May 3,2004, is hereby incorporated by reference in its entirety.

BACKGROUND OF INVENTION

1. Field of Invention

This invention relates to polished polyimide substrates, polymerlaminate structures including polymeric, hybrid organic/inorganic,metallic, and piezoelectric films formed on those substrates, metallizedand stacked polyimide substrates and more particularly to polymerdevices for optical and electronic applications.

2. Discussion of Related Art

Optical waveguide devices are typically made on silicon substrates. Itis desirable that materials used for optical waveguides exhibit certainoptical, thermal and mechanical characteristics, besides low opticalloss. Common silicon micromachining technologies include anisotropic wetetching and dry reactive ion etching (RIE). Passive optical waveguidesexhibiting acceptable losses between 0.1 and 10 dB/cm have beendemonstrated in a number of materials, most notably optical gradeglasses (silica) and PMMA and polystyrene polymers. The highest qualitysilica waveguides with very low losses of 0.1 dB/cm have been depositedon silicon wafers by flame hydrolysis, which yields good control overthe index and thickness of the film but requires heating the porousglass layer to 1250° C. for consolidation. This high temperatureperturbs the crystallographic microstructure of silicon, which affectsits anisotropic wet etching. Furthermore, the flame hydrolysis techniquerequires specialized and expensive equipment and involves the use ofsilane, which is a toxic gas.

The fabrication of ridge waveguides in silica and polymer requiresetching to a depth of several microns. Deep vertical sidewalls with highaspect ratios can be obtained in silicon with RIE. However, RIE is anexpensive process, which requires the use of high vacuum equipment. DeepRIE requires the use of high selectivity gases and appropriate maskinglayers. Silicon dioxide can be used effectively to mask silicon becauseit has a very high selectivity ratio of 200:1 for certain etching gases.This allows etching of several hundred microns deep silicon structurewith an oxide layer of only a few microns. Deep etching of polyimide isproblematic because there is no known masking material with highselectivity ratio toward polyimide. A polyimide substrate cannotwithstand the high temperatures necessary to deposit an oxide film.Photoresist, being a polymer, does not have high selectivity towardpolyimide. Metallic films, such as titanium or aluminum, are usedeffectively to mask polyimide. However, etching of polyimide has beenlimited to a depth of a few microns due to erosion of the metal maskinglayer. The highly energetic plasma ions cause the metallic layer tosputter and deposit metal particles on the polyimide terminating theetching. The lack of suitable masking material for polyimide has been alimiting factor in the use of RIE.

It is desired in certain applications to incline the end faces ofcantilevered film waveguides relative to the axis of the waveguide,especially at air gaps between cantilevered and fixed waveguides. Thiscannot be readily achieved with RIE because the electric field lines ina plasma, which define the trajectory of the energetic ions terminateperpendicularly to the wafer surface. Thus, the desired oblique walls atthe end faces cannot be obtained with silicon micromachining technology.

Silicon micromachined cantilevers carrying film waveguides have made useof films such as silicon dioxide (silica) and nitride. There areproblems associated with fabricating microstructures from the bulk ofsilicon substrates, for example the undercutting of convex corners,which alters the shape of microstructures, e.g. the inertial mass at theend of a cantilever. This prevents the reproducible fabrication ofmicrostructures with 90° corners. This problem can be partiallycorrected with the use of proper corner compensation in the mask layout,however this requires significant experimentation by trial and error todetermine the correct compensation for each mask design. Another problemwith using silica films for waveguides in micro-mechanical applications,which is not encountered in micro-electronic processing, is that thickfilms (up to 50 μm) are needed. The problem with thick films is thatthey tend to crack and peel off due to the large residual stressesbuilt-in during deposition due to the mismatch between the coefficientsof thermal expansion of the film and substrate. Furthermore, thedeposition of silica films is not compatible with silicon micromachiningbecause it requires heating the wafer to a very high temperature, whichmay alter the crystallinity of silicon on which anisotropic etchingdepends. Another drawback of high silica films is the necessity of deepRIE to form ridge waveguides, which is an expensive process and which islimited due to mask erosion.

Certain polymers have been used as waveguide materials. Low loss polymerwaveguides have been most commonly achieved in poly-methyl-methacrylate(PMMA) or polystyrene. However, polymers are affected by bases such asKOH or NaOH, which are used in anisotropic silicon micromachining. Theuse of polyimides on silicon presents problems in regards to wet and dryetching and to the mismatch in the coefficient of thermal expansion, sothat polyimide films on silicon wafers tend to have limited utility infabricating micromachined structures for optical wave guidingapplications.

Polymer film waveguides that are spun cast on planar substrates exhibitthermal and optical properties that are dependent on the depositionparameters. In particular, the degree of anisotropy, such as thedifference between the indices of refraction (birefringence), and thedifference between the coefficients of thermal expansion (CTEs) alongdirections that are perpendicular and parallel to the surface of thesubstrate, depend on the level of stress that is induced in the filmduring fabrication due to the mismatch between the planar CTEs of thefilm and substrate. For mechanical ruggedness and in order to avoidpeeling off or delamination of the film, it is desired to reduce the CTEmismatch as this reduces the level of stress at the interface betweenthe film and substrate. For optical and especially wave guidingapplications, it is desired to reduce the birefringence of the film. Theplanar CTE of a highly anisotropic polymeric film can be as low as 6ppm/° C., while the perpendicular CTE can be as high as 150 ppm/° C. Anisotropic polymeric film has both parallel and perpendicular CTEs about50 ppm/° C. Most polymers have CTEs that are considerably larger thanthat of silicon, which is about 3 ppm/° C. When polymeric films aredeposited on silicon wafers for electronic applications, the planar CTEof the film is chosen as small as possible to minimize the mismatch withsilicon. While this reduces the stresses, it creates a highlyanisotropic film, which is undesirable for optical wave guidingapplications. Thus, it has not been possible to simultaneously reducethe stresses and minimize anisotropy and birefringence in a polymericfilm on a silicon wafer.

The residual side wall angle of a wet etched film is unpredictable dueto the swelling when a developed film dries at elevated temperatures.This is aggravated in multilayered films because the solvents ofsubsequent layers attack the edges of the previous layer at theinterface between the layers resulting in uneven side walls.

The properties of most materials change with temperature. The index ofrefraction is tuned thermally through the thermo-optic coefficient,which is the rate of change of index with temperature. The opticallength of a light path is the product of its physical length times theindex. A change in temperature causes a variation in length due tothermal strain and a change in index due to the thermo-optic effect.Thermal tuning is used in interferometric devices to change the phaseand intensity of light passing through a waveguide. It is desired toachieve temperature-insensitive or athermal design in order to minimizethe dependence of the output of optical devices on environmentaldisturbances, such as fluctuations in temperature. It is important tostabilize the center wavelengths of multi-channel devices such asoptical filters, which tend to drift. Further, athermal design lessensthe dependence on the polarization of light, which is desirable. Thishas necessitated the use of temperature control units, which utilize aheater or a Peltier cooler to maintain the temperature of the deviceconstant. Temperature control requires constant electrical powerconsumption of a few watts and dedicated electronic circuits, which iscostly and undesirable. Uncontrolled athermal operation is achieved bybalancing the effects of the variations of index and length so that thenet change in optical path length is zero. This can be achieved byequating the thermo-optic coefficient to the negative of the product ofthe index of the film times the CTE of the substrate, assuming that thefilm is sufficiently thinner than the substrate. This requires filmswith negative thermo-optic coefficients. Some polymer waveguidematerials, such as fluorinated acrylates, have a negative thermo-opticcoefficient, which is approximately equal to the product of the index ofthe film and the CTE of polymer substrates. Thus, athermal design can beachieved with the use of certain polymer films and substrates. However,it is not possible to tune such an optical device thermally due to itstemperature-insensitive design.

Quartz and ceramic substrates, such as alumina (Al₂O₃) are used for RFapplications. Metal conductor lines are deposited on the substrates formicrowave transmission. The substrate is polished to reduce the loss atthe substrate/metal interface. Both materials have extremely lowdissipation factors, also known as loss tangent. Quartz has a dielectricconstant of about 3.8, while alumina is about 9.9. There is constantdrive in the industry to use higher frequencies beyond 30 GHz into themillimeter wave range. As the frequency increases a larger portion ofthe wave travels in the substrate, a phenomenon known as skin effect. Asubstrate with a certain thickness can support multiple modes at higherfrequency. It is desirable to transmit only the fundamental mode. It ispreferable to limit the thickness of the substrate to a maximumcorresponding to the cut-off of higher order modes. The maximumthickness for single mode transmission depends on the dielectricconstant of the substrate. An alumina substrate, for example, should notbe thicker than 250 μm for frequencies above 10 GHz. The use of a verythin substrate is undesirable because it is fragile. A substrate with alower dielectric constant, such as quartz, can be thicker, for example500 μm while supporting a single mode at the same frequencies. It ispreferable to use thicker substrates with low dielectric constant in therange of 3 to 4 for higher frequencies because it is cheaper tofabricate and easier to handle. However, the use of quartz for RFsubstrates has been problematic because it is expensive and brittle.

When a silicon wafer carrying a polymer film is cut or cleaved, thepolymer film tends to lift off and hang over the cut edge of the wafer.The width of the lifted-off regions can extend up to 300 μm inward fromthe edge. This necessitates removing the entire lifted region of thefilm, for example by ablating with a laser to improve coupling of lightin and out of the waveguide. However, this is problematic because itcreates a relatively long step that the light must traverse between theedge of the wafer and the edge of the film. If this step is at the inputedge of the waveguide where light is focused as a cone or wedge then asubstantial portion of the light can be blocked off. If the step is atthe output edge then it interferes with the collection of the light by alens for feeding into a pick up fiber. This step is particularlyproblematic over silicon wafers. It was necessary to control the endface of a polymer channel waveguide within 5 μm from the cleaved siliconsubstrate edge in order to achieve acceptable coupling of the light (J.C. Chon and P. B. Comita, “Laser ablation of nonlinear-optical polymersto define low-loss optical channel waveguides”, Opt. Lett. 19, 1840,1994). The cleavage of the silicon wafer must be done very carefully sothat the least amount of film is peeled off at the cleaved edges.

To couple light in and out of single mode channel waveguides single modeoptical fibers are typically attached to the end of the waveguides. Thisrequires alignment of the axes of the fiber and waveguide with submicronaccuracy. For example, V-grooves can be etched in silicon substrates andthe alignment between the fiber and waveguide is adjusted while activelymonitoring the coupling efficiency. At the point of maximum efficiency,the fiber is attached to the substrate. It would be desirable to couplelight efficiently between single mode fiber and waveguide passivelywithout monitoring the light intensity during the attachment.

Micro-electro-mechanical (MEMS) devices are fabricated on silicon waferseither by surface micromachining of thin layers deposited on the siliconsubstrate, or by bulk micromachining of the MEMS structure in thesilicon wafer. Bulk-micromachined structures are larger, sturdier andhave higher resonant frequencies. Surface-micromachined structures aresmaller, flimsier and have lower resonant frequencies. However,bulk-micromachined structures require more driving force and power tomove or bend using thin actuating films.

Piezoelectric films, such as PZT or ZnO or AlN, are useful to actuatesurface or bulk micromachined MEMS structures, such as cantilevers. Itis also desirable to etch deep microstructures with high aspect ratiosfor the fabrication of micromechanical devices, such as accelerometersand optical switches. The piezoelectric films are layered betweenmetallic films, such as Pt or Al, which form the electrodes. Thepiezoelectric films are patterned along with the metallic films. RIE hasbeen used to pattern ZnO; and Argon ion beam milling has been used topattern PZT, both of which are expensive dry etching techniques. Aproblem with dry etching has been poor selectivity, i.e. the etch rateof the masking layer or other layers in the structure can be comparableto or even exceed the etch rate of the layer that is intended to beetched. This causes low yield and poor dimensional control of ZnOdevices, and poor selectivity toward PZT relative to the metalliclayers. It also limits the etch depth of MEMS devices to the thicknessof the mask layer, and necessitates the use of extra masking layers,which is undesirable.

Polyimide substrates can be used to package microelectronic components,such as chips. Other materials are currently used to accomplish thistask including organic and ceramic substrates. As chip functionalitybecomes more sophisticated and pin count increases, it becomes necessaryto provide higher density interconnects among the chips. It is oftennecessary to route the signals through multi-level substrates in orderto avoid wire crossings. This is accomplished by drilling vertical viasin each of the layers of the substrate, which are covered by contactpads on each side of the wafer. Higher density is achieved by reducingthe cross-section of the metallic traces in each layer and by reducingthe diameter and spacing between holes. Microvias are commonly drilledwith lasers, such as excimer or tripled YAG. Very narrow holes withdiameters down to 25 μms can be achieved routinely. However, it has beendifficult to coat the cylindrical wall of a hole with an aspect ratiogreater than 1:1 reliably.

Organic substrate fabrication has traditionally consisted of alamination of several sheets of organic materials, such as FR-4 epoxy.FR-4 layers are typically between 1 and 3 mils (25-75 μms) thick. Thelaminate is traditionally drilled using conventional mechanicaldrilling. One drawback of this technique is that the holes end up at thesame locations in all the layers, which wastes board space and preventsthe achievement of high density. Further, the different layers of theassembly must have individual hole patterns because the signal routingrequirements change from layer to layer. This can be accomplished bydrilling the FR-4 layers separately with laser prior to lamination.However, this technique also yields low density because it is difficultto maintain a high degree of alignment between the holes during thelamination process.

Another substrate fabrication technique, which is used widely withceramic substrates, is the build up process. Starting with a rigidceramic wafer, successive dielectric and metallic layers are added usingthick film technology. Each layer is patterned to create eitherhorizontal metal traces or drilled vertical vias. A thick ceramic pastewith metal fillers, such as Fodel manufactured by DuPont, is driven intothe tiny holes using a process similar to screen-printing. The substrateis subsequently co-fired at very high temperature to sinter thematerial. The drawback of the build up process is that the successivelayers take the shape of the layers underneath with the resulting lossof planarity and registration accuracy, hence density.

In order to increase the density of the interconnect it is necessary tofabricate smaller diameter holes and to pack them closer together.However, the challenge is not drilling smaller holes. Holes smaller than50 μm are drilled routinely with laser. The challenge has been coatingthem with metal to ensure reliable electrical connection between bothsides of the substrate. Getting the walls of the tiny holes to wet forthe metal to stick to it has proved to be challenging. There are twomain parameters, which play a vital role in determining the success ofthe metallization, namely the diameter of the hole and its aspect ratio,i.e. the ratio of the depth of the hole to its diameter. The currentlimitation in hole diameter is about 100 μms and the highest aspectratio that can be successfully coated is about 1:1 or even less.

A seed layer is a precursor for growing a metallic layer on any surface.However, the chemical processes responsible for seeding thin metallicfilms on flat surfaces are totally different from those that are used tocoat vertical cylindrical walls of tiny holes. For example, dry coatingtechniques such as sputtering and thermal evaporation yield excellentfilm coverage on flat surfaces but cannot coat narrow holes,particularly those with high aspect ratios. A wet technique, which iswidely used in the semi-conductor packaging industry for metallizing 100μm vias in rigid FR-4 boards, is electroless copper plating. However,electroless plating cannot be used to coat 50 μm or smaller vias becauseit releases hydrogen bubbles of about the same diameter, which gettrapped in the holes and block the plating process. In the case ofceramic substrates the thick Fodel paste cannot penetrate a 50 μm hole.For these reasons, the smallest hole diameter that can be successfullycoated in either material is currently limited to about 100 μms; and thepitch, i.e. center-to-center between holes or pads is limited to about200 μms.

Furthermore, it is desired to eliminate the adhesive layer betweensuccessive layers of a multi-layered substrate. The use of adhesivelayers, such as Pyralux manufactured by DuPont, increase the complexityof the assembly and can become the bottleneck limiting the speed anddensity of the interconnect.

It would therefore be desirable to provide a flexible polyimidesubstrate and a polymer laminate wherein the materials used for thedifferent layers are highly compatible in terms of thermal, mechanical,chemical and machining properties.

It would also be desirable to cost-effectively fabricate, for example,by laser machining in a polymer or a polymer laminate a micro-structure,for example, a cantilevered waveguide.

It would also be desirable to fabricate an opto-mechanical device, suchas an accelerometer or optical switch incorporating a micromachinedcantilevered waveguide.

It would also be desirable to fabricate a micro-mechanical device in aflexible polyimide substrate, which can be actuated with low electricalpower.

It would also be desirable to couple light efficiently and passivelybetween a single mode fiber and a single mode waveguide.

It would also be desirable to provide a multi-layered polyimidesubstrate with a three-dimensional high density interconnect consistingof holes less than or equal to 50 μm diameter and pitch less than orequal to 100 μms, and to eliminate the adhesive layers betweensuccessive layers of the substrate.

SUMMARY OF INVENTION

This invention is directed to polished polyimide substrates for opticalapplications, and to laminates and stacks of wafers fabricated using thepolished substrates.

According to one aspect of the invention, a polyimide substrate has oneor two polished sides with a surface roughness between about 0.25μ inchand about 100μ inch. A polymer waveguide layer can be disposed on apolished side of the polyimide substrate, with the polymer waveguidelayer having a refractive index that is greater than a refractive indexof the polyimide substrate and a thickness so as to support at least oneguided mode in the polymer waveguide layer. A first polymer claddinglayer can be disposed between the polyimide substrate and the polymerwaveguide layer, with the first polymer cladding layer having arefractive index that is smaller than the refractive index of thepolymer waveguide layer. A second polymer cladding layer can be disposedon top of the polymer waveguide layer, with the second polymer claddinglayer having a refractive index that is smaller than the refractiveindex of the polymer waveguide layer.

According to another aspect of the invention, a laminate has at least apolished polyimide substrate and a polymer, or a polyimide or a hybridorganic/inorganic film deposited on the substrate.

According to another aspect of the invention, the laminate can alsoinclude a ceramic layer, preferably a piezoelectric layer, such as PZTor PLZT, AIN, or ZnO on a polished polyimide substrate.

According to another aspect of the invention, the laminate can alsoinclude metallic layers on a polished polyimide substrate.

According to another aspect of the invention, the laminate can include apolycrystalline copper-indium-gallium-diselenide (CIGS) film on apolished polyimide substrate.

According to another aspect of the invention, a method is disclosed forforming a polymer waveguide structure on a polymer substrate. A firstshape of the optical device is defined in the polymer waveguidestructure using a first laser beam emitting in the IR spectral range,and a second shape of the optical device is defined in the polymerwaveguide structure using a second laser beam emitting in the UVspectral range. The first laser beam separates the polymer waveguidestructure at least partially from the polymer substrate. The secondlaser beam produces a gap between the at least partially separatedpolymer waveguide structure and a remaining portion of the polymerwaveguide so as to form a cantilevered waveguide structure. The end faceof the cantilevered waveguide structure facing the gap may beperpendicular or inclined with respect to a surface normal of thepolymer substrate.

In one embodiment, the first laser beam impinges in a first area on abackside of the polymer substrate opposite the polymer waveguidestructure, causing ablation of the polymer substrate in the first areawithout ablating the polymer waveguide structure. The second laser beamimpinges on the polymer waveguide structure in a second area overlappingwith, but smaller than the first area, causing ablation of the polymerwaveguide structure and forming an air gap, thereby releasing thecantilever. The released cantilever can pivot about a fixed end locatedopposite the air gap.

According to yet another embodiment of the invention a method isdisclosed for forming a groove in a polymer laminate which includes anoptical waveguide on a polyimide substrate for coupling light to anoptical fiber. The method includes ablating a groove in the polyimidesubstrate substantially collinear with the optical waveguide. The groovehas a bottom so that a center of the optical fiber inserted in thegroove and contacting the bottom is substantially coincident with thecenter of the optical waveguide in a direction normal to the surface.The sidewalls of the groove are ablated smooth and vertical by adjustingthe ablation parameters of the excimer laser and the design of theoptical delivery system, to securely hold the optical fiber in thegroove.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings, are not intended to be drawn to scale. In thedrawings, each identical or nearly identical component that isillustrated in various figures is represented by a like numeral. Forpurposes of clarity, not every component may be labeled in everydrawing. In the drawings:

FIG. 1 a shows schematically a top view of a cantilever structure beforerelease, machined in a VESPEL® substrate using a CO₂ laser;

FIG. 1 b shows schematically a cross-sectional view of the cantileverstructure of FIG. 1 a taken along the line IIIb-IIIb;

FIG. 1 c shows schematically in cross section ablating with an excimerlaser a gap for releasing the cantilever structure of FIG. 1 a, with thecross section taken along the line IIIc-IIIc of FIG. 1 d;

FIG. 1 d shows schematically a top view of the cantilever structure ofFIG. 1 a after release by ablation of the gap with an excimer laser;

FIG. 2 is a schematic diagram of a phase modulation in radians vs. RMSdrive voltage applied to a piezoelectric plate;

FIG. 3 shows schematically a pattern of cantilevers cut in a polishedVESPEL® wafer using a CO₂ laser; and

FIG. 4 shows schematically a cross-sectional view of an optical fiberlocated in a groove laser-machined in a polyimide/polymer laminate.

FIG. 5 shows schematically a perspective view of a cantilever with twosections of a piezoelectric laminate divided along a neutral axis of thecantilever.

FIG. 6 shows an array of 50 μm holes on 100 μm pitch drilled withexcimer laser.

FIG. 7 shows an array of 50 μm holes on 60 μm pitch drilled with tripledYAG laser.

DETAILED DESCRIPTION

This invention is not limited in its application to the details ofconstruction and the arrangement of components set forth in thefollowing description or illustrated in the drawings. The invention iscapable of other embodiments and of being practiced or of being carriedout in various ways. Also, the phraseology and terminology used hereinis for the purpose of description and should not be regarded aslimiting. The use of “including,” “comprising,” or “having,”“containing”, “involving”, and variations thereof herein, is meant toencompass the items listed thereafter and equivalents thereof as well asadditional items.

This invention provides materials and methods suitable for fabricationof a laminate comprising a waveguide and a polymer structure disposed ona flexible polyimide substrate. The invention also provides amicromachined cantilever supporting an optical waveguide, as well as alaser micromachining process, which allows fabrication of the desiredwaveguide end face inclination by directing an ablating laser beam ontothe polyimide substrate at a predetermined angle with respect to anormal to the surface of the substrate. The combination of materialsdisclosed herein achieves advantageous optical, mechanical, processingand thermal properties.

This invention uses laser micromachining for the fabrication ofmicro-electro-mechanical systems (MEMS) andopto-micro-electro-mechanical systems (OMEMS), as a replacement forchemical or physical etching. This process is cost-effective and avoidswet or dry etching in the fabrication of microstructures.

Polyimide substrates suitable for use in this invention are flexiblewith a modulus of elasticity ranging between 1 and 10 GPa. Thesepolyimide substrates are generally at least 200 μm thick, but thinnermembranes can be used. The polyimide material can preferably withstandtemperatures such as those encountered in cutting and polishing withoutsignificant degradation of the physical and mechanical properties. Thesurface of the polyimide material can be polished to achieve a surfaceroughness in the range between about 0.25μ inch and about 100μ inch.VESPEL® is a suitable polyimide material commercially available fromDuPont, which can be machined into a suitable substrate and which canoperate continuously from cryogenic temperatures to 288° C. in air, to315° C. in inert environments or vacuum, and can withstand temperaturesup to 482° C. intermittently. It is tough yet compliant, which makes itsuitable for a flexible substrate. Because of its inherent mechanicalstrength, stiffness and dimensional stability at machining temperatures,VESPEL® can be machined with the use of standard metalworking equipment.It can also be ground, buffed, lapped and polished. VESPEL® has adielectric constant of 3.4.

VESPEL® is a mechanically sturdy, visually opaque, brown coloredpolyimide which is available commercially in the form of circular discswith diameters varying between about 2 inch and about 16 inch, andthickness between about 0.25 inch and about 4 inch. It is also availablein blocks up to 2 inch thick and up to 13 inch wide, which can bemachined into plates or cylinders. These disks are too thick to be usedas wafers. In the present invention VESPEL® polyimide is cut from bulkcylinders, preferably with a wire and polished to form thin wafers orsubstrates, on which films are deposited. VESPEL® wafers with thedesired final thickness can also be directly formed by molding to athickness close to the final thickness and polishing the surface withoutthe need for cutting from a thicker disc. VESPEL® wafers of anythickness are obtained by cutting and polishing the VESPEL® discs. Thethickness of the wafer can vary along the radius. For example, the innerportion around the center of the wafer can be thin as a membrane whilethe outer portion near the perimeter of the wafer can be thicker as aring, or vice versa. VESPEL® polyimide is also available in other forms,such as rods, plates, plaques, and bars, which can be used to formsubstrates. VESPEL® bulk forms which are made from either SP, SCP or STpolyimide resins can be used to form substrates useful for thisinvention. The SCP resin has a water absorption coefficient of about0.08% by weight in a 100% relative humidity environment. VESPEL® withfillers such as 15% or 40% graphite, and 10% Teflon can also be used toform substrates useful for this invention. VESPEL® has a coefficient ofthermal expansion varying between 20 and 54 ppm/° C., preferably in therange 40 to 54 ppm/° C. Polyimides other than VESPEL® but with similarproperties are also suitable for making polished wafers. Thermoplasticpolyimides can be used to make polished wafers if a certainpredetermined temperature known to cause the thermoplastic to flow isnot reached during the cutting, polishing and subsequent operation ofthe wafer. Thermoplastic polyimide wafers with near final dimensions canbe injection molded and then polished. It is preferred to use thermosetor thermoset-like behaving polyimides to polish wafers suitable for thisinvention.

Polyimide discs with diameters varying between about 2 inch and about 6inch and thickness varying between about 0.25 inch and about 4 inch weresliced to form thin wafers with thickness varying between about 500 μmand about 1.25 μm. The wafers were subsequently polished on either oneor both sides to an average surface roughness of about 0.25μ inch. Thepolished wafer thickness was about 200 μm, 500 μm, 625 μm, 750 μm, 875μm, 900 μm, and 1 mm. This yielded flat polyimide wafers with uniformthickness and highly reflective surfaces ready for film deposition.Alternatively, polyimide wafers can be ground using a diamond wheel.Polishing of the polyimide surface improved the reflectivitysignificantly. Polyimide wafers can be polished to an average surfaceroughness of 0.025μ inch with the use of chemical mechanical polishing(CMP). A polished polyimide wafer can be bonded to a silicon wafer.

Laminate structures can be deposited on polished polyimide substrates.The laminate can consist of one or more films or layers. Suitablelaminate materials are polymers, polyimides, hybrid organic/inorganiccompounds, metals, ceramics, and piezoelectrics. Polymeric films thatcan be deposited on polished polyimide substrates include acrylates,polyimides, polycarbonates, cyclobutanes, polyetheremide, SU-8, andPDMS. Suitable waveguide materials that can be deposited on polishedpolyimide substrates are perfluorinated polymers, such asperfluorocyclobutane (PFCB); halogenated acrylates, such as PMMA;fluorinated and non-fluorinated polyimides; and hybrid organic/inorganiccompounds, such as silicate-based spin-on glasses that are depositedusing the solgel process. Fluorinated polymers contain the C—F molecule,which is known to produce low optical losses in the NIR region of thespectrum.

Laminate waveguide structures were deposited on polished VESPEL®substrates using organic materials, in particular polyimide films, witha glass transition temperature greater than 200° C., and having athickness between 0.1 μm and 15 μm. Multi-layer films can be deposited.These polyimide films were spun cast from solutions directly on theVESPEL® substrate. In some cases an adhesion promoter layer was usedbetween the polyimide film and the polyimide substrate. Other depositionmethods include, for example, dipping, spraying, coating, or gluing.Polymeric waveguides can be obtained as freestanding films, which can beglued onto the polyimide substrate. Channel waveguides can be fabricatedin polymeric films by wet or dry etching. Channel waveguides can also befabricated in photosensitive polymeric films through development.Photosensitivity in polymeric films can be obtained through theinclusion of dopants, such as alkylated groups or chromophores. Uponexposure to UV light, the dopant cross-links to the polymer matrix,thereby changing the index. A channel waveguide having a desiredcross-section can be fabricated in photosensitive polymers withoutetching or material removal by doping in an amount sufficient to achievea controlled increase in index in the core region relative to thesurrounding regions upon exposure to UV light.

Polyguide™ films, manufactured by DuPont in Wilmington, Del., containlow molecular weight acrylate and methacrylate monomers. These monomersdiffuse within a polymer binder matrix and polymerize when exposed to UVlight thereby changing the index of refraction of the polymer.Waveguides and in particular single mode waveguides are fabricated byphotochemical polymerization of the monomers with the use of laserwriting or photolithography. Channel waveguides can be fabricated inPolyguide sheets, without etching or material removal, by exposing itselectively through a mask to UV radiation. Several meters long singleor multimode waveguides can be fabricated in Polyguide polymer sheets orrolls. Freestanding polyguide sheets can then be cut and glued ontopolished polyimide substrates.

A laminate has at least a polished polyimide substrate and a hybridorganic/inorganic waveguiding film based on an Si—O—Si backbonedeposited on the substrate using the solgel process. Theorganic/inorganic hybrid can be fluorinated. These hybrid materials havea coefficient of thermal expansion varying between 50 and 250 ppm/° C.An optional optical buffer layer can be used between the wave guidingfilm and the polyimide substrate, and an optional upper cladding layercan be deposited on top of the wave guiding film. Each layer has athickness in the range specified above. The laminate demonstratescompatibility of the mechanical, thermal, chemical and opticalproperties of all the layers and specifically closeness of thecoefficients of thermal expansion of the layers in the laminate, whichminimizes residual stresses. The coefficients of thermal expansion ofthe laminate are preferably in the range between 20 and 75 ppm/° C. Thedeformation and warpage of the laminate and the delamination and/orcracking of the film are thus reduced when subjected to baking cycles. Awave guiding laminate can exhibit minimal stresses and birefringencebecause the coefficient of thermal expansion of the polyimide substrateis about 50 ppm/° C. close to that of isotropic polymeric films.

An athermal design of an optical device can be achieved by using a waveguiding film material, such as fluorinated acrylates, whose thermo-opticcoefficient is approximately equal to negative of the product of theindex of the film and the CTE of the polished polyimide substrate.Reduced thermal sensitivity can be achieved by using a wave guiding filmwhose thermo-optic coefficient is close to the negative of the productof the index of the film and the CTE of the polished polyimidesubstrate. The optical device can be tuned mechanically with the use ofa MEMS structure, such as a cantilever, without disturbing its athermalbehavior, and without the need for temperature control, because MEMSactuation dissipates very little electrical power, on the order ofμWatts. A displacement of the cantilever causes a change in theintensity or length of the optical path of the light passing through thewaveguide.

The laminate can also include metallic films, such as aluminum (AL),copper (Cu), gold (Au), silver (Ag), titanium (Ti), nickel (Ni),platinum (Pt), molybdenum (Mo), chromium (Cr) on a polished polyimidesubstrate. The metallic layers can be deposited either by sputtering,evaporation, electroplating or electroless plating. Several metallicfilms were deposited on polished VESPEL® substrates by sputtering and byevaporation. The films adhered well to the polished VESPEL® substrates.The VESPEL® substrates did not outgass an amount sufficient tocontaminate the sputtering ovens. A laminate can include one or moremetallic films that exhibit low loss at RF and millimeter wavefrequencies disposed on a polished polyimide substrate.

The laminate can also include a ceramic layer, preferably apiezoelectric layer, such as PZT or PLZT, AlN or ZnO on a polishedpolyimide substrate. A first metallic layer, preferably platinum, isdisposed on a polished polyimide substrate. A piezoelectric layer,preferably PZT, is deposited on the first metallic layer. A secondmetallic layer is deposited on top of the piezoelectric layer. PZT canbe deposited using the solgel process. The two metallic layers serve aselectrodes. A voltage is applied across the electrodes, which causes alateral displacement of the piezoelectric film through the piezoelectriccoefficient d₃₁. The flexibility of the polyimide substrate allows thethin piezoelectric layer to bend the substrate with reasonable appliedvoltages that are compatible with commonly available driving circuits.This allows a MEMS structure, such as a micromachined cantilever, whichcan be as thick as the substrate itself, to be actuated by a thinpiezoelectric layer deposited on top of the polyimide substrate. Thepiezoelectric layer can be deposited between the polished polyimidesubstrate and the wave guiding film, or it can be deposited on top ofthe wave-guiding film. The metallic and ceramic films can be patternedby laser ablation, which eliminates the problems associated withselectivity of wet or dry etching among the layers.

The back side of a VESPEL® wafer can be polished. Polymeric, polyimide,hybrid organic/inorganic, metallic, and piezoelectric laminates can bedeposited on the polished backside of a VESPEL® wafer. Deposition ofsimilar laminates on both sides of a VESPEL® wafer preserves thesymmetry and reduces the tendency for warpage.

A micromachined cantilever is fabricated in a polished polyimidesubstrate. A laminate consisting of a first metallic layer, apiezoelectric film, and a second metallic layer is deposited on thepolished polyimide substrate. The laminate is ablated along a neutralaxis of the cantilever to create two electrically isolated sections ofthe laminate, a left section and a right section, which can be drivenindependently. The two sections can have a common electrical ground.When the sections are driven with different voltages unequal strainsdevelop in the left and right portions of the cantilever, which causethe cantilever to displace in a direction parallel to a surface of thesubstrate.

The laminate can include a polycrystallinecopper-indium-gallium-diselenide (CIGS) film on a polished polyimidesubstrate, such as used in solar cells.

It is necessary to prepare the input and output end faces of waveguiding films on polyimide substrates to couple light in and out of thewaveguide, and to prepare the air gap between a cantilevered waveguideand a fixed waveguide. The input and output end faces can be cut, forexample by dicing with a diamond blade, or ablated with an excimerlaser. The dicing technique can be more cost-effective for preparing theinput/output edges of the film. The narrow air gap of only a few micronsbetween two waveguide end faces is fabricated by laser ablation.

For the preparation of optical quality input and output waveguide edgesat air/film interfaces, a light source whose spot size can be focused toless than about 10 μm, such as an excimer laser is used. This yieldssmooth edges, which reduce scattering of the light. Polymer materialsabsorb at UV wavelengths, therefore any laser or light source whichemits in the UV, or whose frequency can be doubled or tripled orquadrupled to yield a wavelength in the UV range between 180 nm and 400nm, and whose spot size can be focused to less than about 10 μm can beused. However, for the preparation of the edges of films that are notcrossed by light, or for the fabrication of coarse contours of MEMSstructures, or for etching blind or via holes through the substrate, anIR laser, such as a CO₂ or YAG laser that etch polymer materials fasterthan the excimer laser, can be used. The quality of the IR laser cut canbe improved with the use of a short pulse high-energy laser such as aQ-switched CO₂ laser. The air gap walls and input/output edges of waveguiding films can be ablated smooth and planar by adjusting theintensity profile and parameters of ablation of the excimer laser. Theablated film edges are slightly tapered at a half-angle of about 0.75°to the vertical. The taper angle depends on the design of the opticaldelivery system. The etch rate and quantity of debris released byablation depend on the fluence of the laser and the pulse repetitionrate. These parameters are optimized to yield controllable etch ratewhile minimizing the effect of debris on the wave guiding film. Theattachment of the debris to the film can be reduced with the use ofsacrificial coatings.

EXAMPLE 1 Air Gap Ablated with Excimer Laser

Air gaps of widths 3, 5, 7 and 10 μms were ablated in a polyimide film,about 4 μm thick, disposed on a polished VESPEL® wafer with a LambdaPhysik ArF laser emitting at 193 nm. The laser was pulsed at the rate of40 Hz and had an energy density of 5.679 J/cm². The fluence, repetitionrate, pulse width and total number of pulses can be controlled with acomputer. The laser beam was homogenized with a lens array, whichoverlaps four beams to provide a single image with a square-topintensity profile. The wafer was mounted on a computerized translationstage and scanned in its plane during the ablation to create air gaps ofdifferent lengths. The laser beam was directed either normal to thesurface of the wafer or inclined by an angle of 30° relative to thenormal to ablate vertical or inclined air gaps, respectively. Thesidewalls obtained with laser ablation were smooth and planar. Thefluence and pulse repetition rate of the excimer laser influence thequantity of debris released by the ablation.

A polyimide wafer carrying a film can be cut into many pieces by dicingwith a diamond blade, or cutting with a laser such as CO₂ or YAG, orexcimer laser. The blade and CO₂ laser separate the wafer. The depth ofthe cut can be controlled more precisely with the excimer laser. Inorder to separate a wafer it is not necessary to ablate through thethickness of the wafer with the excimer laser. It is often sufficientand more economical to ablate through the film and to continue theablation to a certain depth, for example about 100 μm into the polyimidewafer. Structural weakening of the wafer along the scribing line allowscleaving of a polyimide wafer in a manner similar to a silicon wafer.Cleaving by hand is a simple and cost-effective method of breaking alaminate into many pieces.

A deep vertical step was ablated in a VESPEL® wafer near the edge of apolymer film with an excimer laser. The depth of the step necessary toclear the path of the light depends on the numerical aperture of thefocusing optics. A depth of about φμm is sufficient to clear the path ofthe focused light butt-coupled to the input edge of the waveguide. AVESPEL® wafer was cleaved first by scribing with an excimer laser andthen breaking by hand. This procedure for separating a VESPEL® waferavoids lifting the film off the edge of the substrate. The quality ofthe ablated edge of the film was sufficient to couple light into thewave guide without further smoothening of the edge.

To fabricate a structure, such as a cantilever, a polyimide wafer iscut, preferably using a pulsed CO₂ laser. A cantilever can also befabricated using an excimer laser. The pulse width of the CO₂ laser isabout 0.5 ms. The CO₂ laser is controlled with a computer, which storesAutoCad data representing the contour of the cantilever. The cantilevercontour was cut in a single traversal of the wafer by the CO₂ laserbeam. The minimum spot size achievable with a CO₂ laser is typically atleast about 50 μm, which tends to produce ragged edges in the waveguidefilm and the micromachined cantilever. Smoother edges can be obtainedwith the use of a Q-switched CO₂ or excimer laser.

Most of the contour of the cantilever is fabricated with the CO₂ laser,except for a narrow region at the location where the air gap issubsequently formed with an excimer laser. The CO₂ laser does notseparate the cantilever completely from the wafer but keeps it suspendedat the narrow region. The narrow region in the VESPEL® wafer was ablatedwith excimer laser concurrently with the formation of the air gap.Ablation of the air gap in the organic film and concurrent ablation ofthe VESPEL® material underneath it releases the cantilever, which thencan freely move due to acceleration or applied force.

Debris produced by CO₂ laser cutting can deposit on the laminate andinterfere with its wave guiding properties. This can be prevented byflipping the VESPEL® wafer upside down, so that the CO₂ laser beamimpinges on the uncoated back surface of the VESPEL® wafer. In this way,the CO₂ laser beam cuts through the bulk of the VESPEL® wafer firstbefore reaching the waveguide film. Any remaining debris, which depositon the waveguide end faces can subsequently be removed by excimer laserablation. It is preferred that the step of CO₂ laser cutting precede thedicing and excimer laser ablation steps. It is also preferred that thelast step in the fabrication process be the excimer ablation step.

The procedure for releasing the cantilever and forming the air gaputilizes two consecutive steps:

-   -   (i) ablating from the uncoated back surface of the VESPEL® wafer        to a depth short of ablating through the whole wafer and a width        larger than the desired width of the air gap, and    -   (ii) ablating from the side carrying the wave guiding film to        the remaining depth of the wafer and a width equal to the        desired air gap. To accommodate a larger tolerance in the        placement of the air gap within the ablated area across the        visually opaque VESPEL® wafer, the area ablated from the        uncoated back side of the wafer is made wider than the desired        air gap width.

Accordingly, the fabrication of a cantilever includes: cutting the shapeof the cantilever with CO₂ laser without releasing it, ablating thenarrow region of the VESPEL® wafer with excimer laser and ablating theair gap concurrently to release the cantilever, and ablating theinput/output edges of the waveguide film with an excimer laser, notnecessarily in this order.

EXAMPLE 2 Laser Micromachined Cantilever Beam with Ablated Input/OutputEdges

A cantilevered waveguide was fabricated in a polished VESPEL® waferaccording to the procedure outlined above by following the sequentialsteps of:

-   -   (a) forming an unreleased cantilever contour by cutting the        VESPEL® wafer with a CO₂ laser,    -   (b) ablating the input/output edges of the waveguide film with        an excimer laser, and    -   (c) ablating the narrow region of the VESPEL® wafer with the        excimer laser and ablating the air gap concurrently to release        the cantilever.

The cantilevered waveguide of example 2 is shown schematically in FIGS.1 a-1 d. The polished VESPEL® wafer is 25 mils (625 μms) thick. The linedrawings were prepared from an SEM image. The main cantilever sectiondenoted by 41 in FIGS. 1 a and 1 d is 50 mils long by 25 mils wide. Thearea 42 representing the inertial mass is 110 mils long by 70 mils wide.As indicated in FIG. 1 b, a CO₂ laser directed from the back side of thewafer creates most of the contour of the cantilever except for anarrow—approximately 10 mils wide—region 43 at the location where theair gap is subsequently formed by excimer laser ablation. The CO₂ laserdoes not release the cantilever but keeps it suspended from the narrowregion 43. The input edge of the waveguide film is denoted by 45 and theoutput edge of the waveguide film is denoted by 46. After formation ofmost of the contour of the cantilever (except the narrow region) with aCO₂ laser, the wafer is ablated with an excimer laser from the uncoatedbackside at region 43 to a depth of 550 μm and width of 50 μm. The waferis then ablated with an excimer laser from the front side carrying thewaveguide, as indicated in FIG. 1 c, to a depth of 75 μm and a widthequal to that of the desired air gap. The total ablation depth is equalto the thickness of the wafer. The cantilever is released byconcurrently ablating the air gap in region 43 of the waveguide film andthe VESPEL® material underneath. The portion of region 43 adjacent tothe output edge 46 is not necessary. The light exiting the cantileveredwaveguide can be picked up directly by a fiber or adjacent waveguide.

EXAMPLE 3 Laser Micromachined Cantilever Beam with Diced Input/OutputEdges

A cantilevered waveguide similar to that given above in Example 2 wasfabricated in a polished VESPEL® wafer, except that the input/outputedges of the waveguide film in step (b) were diced instead oflaser-ablated (not shown).

The displacement of the cantilever causes a change in the length of thepath of the light passing through the cantilevered waveguide, which ismeasured interferometrically. In an exemplary measurement, a section ofstraight planar waveguide incorporating a micromachined cantilever isinserted in one arm of a fiber optic Mach-Zehnder interferometer. Thecantilever is driven with a piezoelectric sheet made from PZT materialto simulate acceleration. The piezoelectric plate is drivenlongitudinally near its resonance to attain maximum displacement. Theresonant frequency of the piezoelectric plate was 11.523 kHz.Application of a sinusoidal voltage to the piezoelectric plate generateddynamic displacements of the cantilever, which were picked up as opticalphase change by the interferometer. FIG. 2 displays a graph of the phasechange in radians vs. the drive voltage applied to the piezoelectricsheet. A linear fit to this plot provided the value of the phaseshifting coefficient, which is the phase change per unit voltage. Avalue of 1.281 rad/V_(rms) was determined at f=11.523 kHz, correspondingto an optical path length change of 0.127 μm per μm of lateralcantilever tip displacement.

The cantilevered waveguides of Examples 2 and 3 can be used, forexample, in an interferometric optical accelerometer, or an opticalswitch for routing of light in optical telecommunications networks.

FIG. 3 shows a pattern of adjacent cantilevers 51 cut with a CO₂ laserand released with excimer laser, in a polished VESPEL® wafer 52. The CO₂laser cuts the entire pattern of cantilevers in a single traversal ofthe wafer. The wafer is scanned relative to the stationary CO₂ laser.The excimer laser is stepped and repeated across the wafer area torelease all the cantilevers in the pattern.

EXAMPLE 4 Fabrication of a Groove for Mounting a Fiber

A groove is fabricated in a polished polyimide wafer by laser ablationto hold a fiber for attachment to a channel waveguide. The groove isparallel and colinear with the channel waveguide. The edge of thechannel waveguide adjacent to the groove is prepared by laser ablation.The fluence of the ablating laser and the optical delivery system can beadjusted to yield a smooth and vertical groove with sub-micron accuracy,which holds the fiber tightly horizontally and vertically. An opticalfiber is inserted in the groove by pressing. It is translated axiallyuntil its tip contacts the edge of the waveguide. The depth and width ofthe groove can be controlled very precisely within about half a micron(or about 0.5 dB optical loss) so that the axis of the fiber corecoincides with the axis of the waveguide. The accuracy of the verticalalignment between fiber and waveguide depends on the accuracy with whichthe groove depth can be controlled. This is very precisely known fromknowledge of the number of pulses and the ablation rate of the materialper pulse. The ablation rate depends on the fluence of the laser, i.e.energy density per pulse. A typical ablation rate for polyimide is about0.5 μm/pulse. For example, if the waveguide core layer is centered at adistance of about 10 μm above the polished substrate surface, thenassuming a fiber radius of 62.5 μm, the bottom of the groove must be ata distance of 52.5 μm below the substrate surface. At a rate ofapproximately 0.5 μm/pulse, it would take about 105 pulses to form thegroove. The accuracy of the horizontal alignment between fiber andwaveguide depends on the accuracy with which the groove width andlocation can be controlled. This is very precisely controlled with theuse of standard photolithographic stepping processes. This yields thedesired accuracy for coupling light between single mode fibers andwaveguides. A cross section of a laminate 80 comprising a groove 81,fiber 82, and waveguide comprising a first optical buffer (lowercladding) layer 83, a second optical wave guiding (core) layer 84, and athird optical (upper cladding) layer 85 on top of a substrate 86 isshown in FIG. 4. The center of the channel waveguide 87 in the corelayer 84 coincides with the center of the fiber 88. The groove is imagedon the laminate through a reticle. The groove can be fabricated with anexcimer laser, which emits pulses typically on the order of nanosecondslong. Alternatively, the groove can be fabricated with lasers, whichemit picosecond or femtosecond pulses for a more precise control of thegroove dimensions.

EXAMPLE 5 Micromachined Cantilever Moving Parallel to Surface ofSubstrate

A metal/piezoelectric laminate 90 disposed on a polished polyimidesubstrate 96 is shown in FIG. 5. A cantilever 95 is micromachined in thesubstrate 96. The laminate 90 is ablated with an excimer laser along aneutral axis of the cantilever 94 to create two sections of laminate 90having a common electrical ground 93, a left section 91 and a rightsection 92. When two different voltages V₁ 97 and V₂ 98 are applied tothe left and right sections 91 and 92, respectively, the cantilevermoves in a direction 99 parallel to a surface of substrate 96.

A polyimide wafer can be drilled with laser to form an array of holes.The holes can have diameters less than 100 μms and center-to-centerspacing less than 200 μms. Typically, holes having a diameter of about50 μms or less and spaced 100 μms center-to-center are drilled in thepolyimide wafer using either excimer laser emitting at 248 nm or 193 nm,or tripled YAG laser emitting at 355 nm to produce microvias suitablefor microelectronic interconnections. Through holes as well as blind viaholes can be fabricated.

EXAMPLE 6 50 μm Diameter Holes on 100 μm Pitch Drilled with Laser

50 μm diameter through-holes on 100 μm pitch (center-to-center) aredrilled through a polyimide wafer using an excimer laser, as shown inFIG. 6. An array of more densely packed holes of the same diameter on a60 μm pitch drilled using a tripled YAG laser is shown in FIG. 7. Thehalf-cone taper angle obtained with the excimer laser is about 0.75°. 25μm diameter holes can be fabricated with either laser. This enables thefabrication of 50 μm pads covering the holes for electrical contact.

The holes are fabricated either sequentially by stepping the wafer underthe focused YAG laser beam, or by imaging the excimer laser through areticle on the wafer. Both techniques achieve an accuracy of a fewmicrons in the registration of the holes. The pads are fabricatedphoto-lithographically with the use of a mask. The overlap between thepads and holes depends on the alignment between the two patterns. Thiscan be achieved with the use of a mask aligner. The use of a thickpolyimide wafer (preferably between 150 and 250 μms) allows fineralignment and better feature registration. This will push the currentpitch limit from 200 μm down to 100 μm.

A typical hole for high density interconnect has a diameter less than 50μms. Thus, the hole has an aspect ratio between 3 and 10:1. Directmetallization, which is an alternative to electroless copper plating, isused to coat the cylindrical walls of the holes. This ensures reliableelectrical connection between both sides of the wafer. This technique isparticularly effective for coating high aspect ratio holes withdiameters of 50 μm or less. The cylindrical wall of the hole is coatedwith a metallic film about 5 microns thick. The roughness of thelaser-drilled hole enhances the adhesion of the metal film to the wall.

A polyimide wafer is cladded on both sides with a metallic film bysputtering or evaporation followed by electroplating. A thin metallicseed layer consisting typically of 300 Angstroms of chromium or titaniumfollowed by 3000 Angstroms of Copper or gold is sputtered on both sidesof the wafer. It is then electroplated with copper up to 1 μm. Equalamounts of metal are removed from each side of the wafer to createmetallic traces while preserving symmetry. The holes can be drilledeither before or after cladding of the polyimide wafer. Carbon debrismay deposit on the surface of the wafer during ablation, which canaffect the adhesion of subsequent layers. The carbon deposit isdissolved with polyvinyl alcohol. Once the holes are drilled, thevertical walls of the holes are ready for direct metallization, which isbased on different chemistry from electroless copper plating. Directmetallization can coat 50 μm holes because it does not cause hydrogenevolution during metallization. Direct metallization uses low viscositysolutions and avoids the use of Formaldehyde, which is carcinogenic.

Direct metallization can be achieved using at least two alternativechemical systems, namely Palladium colloidal or conductive polymer. Inparticular, the polymer, such as the one supplied by Enthone, Inc., WestHaven, Conn., is almost as conductive as copper and specificallydesigned to provide complete coverage of blind vias and high aspectratio holes with 100 μm diameter or smaller. The system activates thecylindrical surface of the polyimide by supplying manganese, which getsabsorbed by the polyimide in the tiny hole. The Manganese is notabsorbed by copper, which covers the horizontal surface of the polyimidewafer. Subsequently, the system supplies a monomer that polymerizes uponreacting with the manganese to create the conductive polymer. Thus, theconductive polymer sticks to the cylindrical wall where the manganesewas absorbed and provides a continuous electrical connection from oneside of the wafer to the other. The success of the direct metallizationprocess hinges on the flow of manganese through the tiny hole. Theroughness of the wall of the hole enhances these chemical reactions.

The direct metallization layer serves as a seed layer on the verticalcylindrical wall, in much the same way the sputtered layer serves as aseed layer on the flat surface. It is only about 0.5 μm thick andfollows the contour of the vertical wall conformally. It has a highelectrical resistance because it is very thin. It must be followed byelectroplating to build up the layer to the desired thickness, usuallybetween 3 and 5 μm, to reduce the resistance. Electroplating can becontinued until the hole is completely filled if so desired.

After successful metallization of the vias, the pads are fabricatedusing either the subtractive or additive process following standardphotolithographic procedures.

The drilled and metallized polyimide wafers are aligned, stacked on topof each other and attached using flip chip bonding techniques to providemulti-layered high density three-dimensional microelectronicinterconnects suitable for packaging of chips. The stacked polyimidesubstrate overcomes the limitations of organic and ceramic substratetechnologies.

EXAMPLE 7 Assembly of Multi-Layered Polyimide Substrate

Polyimide wafers of suitable thickness are cladded with thin metalliclayers as described above. Holes are drilled with laser in each waferseparately. Different wafers can have different thicknesses anddifferent hole patterns. The wafers are metallized using directmetallization and electroplated to create the pads and provide secureelectrical connections across each wafer. The wafers are patterned tocreate horizontal metallic traces and isolate adjacent vertical vias.The wafers are stacked on top of each other in order according to theelectrical circuit layout and aligned using fiduciaries, which arecommonly used in semiconductor wafer alignment. The locations of thepads in two mating surfaces coincide. The pads are attached usingelectrically conductive bumps similar to those used in flip chip bondingfor die attachment. Thus, the successive layers of the substrate can beattached internally similarly to the way the chip is attached to thesurface of the upper layer using stencil printing technology. The use ofconductive bumping technology provides electrical contact as well asmechanical ruggedness and eliminates the need for adhesive layersbetween the successive wafers.

The materials that are commonly used for flip chip bumping areconductive polymer epoxies, which perform simultaneous functions forelectrical connection and mechanical adhesion. In particular, Epotekbrand epoxies, manufactured by Epoxy Technology Inc., Billerica, Mass.,are available and widely used for a variety of flip chip bondingapplications because of its good electrical and adhesive properties.Epotek is a thermoset polymer, which cures at low heat, without the needfor UV radiation, and which does not contain any solvents. Thesefeatures make it attractive for stacking polyimide wafers because it canbe easily cured between two opaque wafers. Stencil printing is aninexpensive technology for depositing polymer bumps, which is widelyused in flip chip bonding applications. Suitable bumps for polyimidewafer attachment can be produced by stencil printing using a stainlesssteel stencil with aperture diameters of about 70 μm. Finer bumps with adiameter of about 50 μm can be obtained with the use of an electroplatedNickel film. A 100 μm pitch can be achieved using stencil printingtechnology. This technology is a low cost alternative to electro-platedsolder and indium bump bonding, which can achieve a smaller pitch below50 μm.

A grid array of conductive epoxy bumps can be placed at 100 μm pitch ona polyimide wafer with a 10 μm placement accuracy. The electricallyconductive bumps provide sufficient mechanical strength and adhesion dueto the significant number of bumps in the array, that additionalmechanical reinforcement may not be necessary. However, supplementalmechanical ruggedness and adhesion between stacked layers can beprovided by stencil printing non-conductive epoxy bumps along theperiphery of the wafer at a coarser pitch outside the area of themicrovias. These bumps would be used only for the purpose of reinforcingthe assembly mechanically but would not provide any electricalfunctionality. Further, the epoxy dispensed around the perimeter wouldnot interfere with the functionality of the polymer bump-bondedmicrovias because it is non-conductive. A combination of conductive andnon-conductive epoxies bumps can be used to assemble the substrate.

The foregoing is considered only illustrative of the currently preferredembodiments of the invention presented herein. Since numerousmodifications and changes may occur to those skilled in the art, it isnot desired to limit the invention to the exact construction used toillustrate the various means comprising the invention.

Having thus described several aspects of at least one embodiment of thisinvention, it is to be appreciated various alterations, modifications,and improvements will readily occur to those skilled in the art. Suchalterations, modifications, and improvements are intended to be part ofthis disclosure, and are intended to be within the spirit and scope ofthe invention. Accordingly, the foregoing description and drawings areby way of example only.

1. A substrate for a circuit structure comprising: a planar substratemass of polyimide material having a first side and a second side, saidfirst side being polished to a surface smoothness between about 0.025μinch and 100 μinch, capable of receiving a circuit structure.
 2. Thesubstrate for a circuit structure of claim 1, wherein said surfacesmoothness is between about 0.025 μinch and 0.5 μinch.
 3. The substratefor a circuit structure of claim 1, wherein said surface smoothness isbetween about 0.25 μinch and 100 μinch.
 4. The substrate for a circuitstructure of claim 3, wherein said surface smoothness is between about0.25 μinch and 0.5 μinch.
 5. The substrate for a circuit structure ofclaim 1, wherein said polyimide is a thermoset polyimide.
 6. Thesubstrate for a circuit structure of claim 1, wherein said polyimide isa thermoplastic polyimide.
 7. The substrate for a circuit structure ofclaim 6, wherein said thermoplastic polyimide is injection molded. 8.The substrate for a circuit structure of claim 1, further comprising afirst layer applied to said first polished side of said planar substratemass, a second layer applied over said first layer, wherein the index ofrefraction of said second layer is greater than the index of refractionof said first layer.
 9. The substrate for a circuit structure of claim8, wherein said first polymer layer and said second polymer layer arefluorinated polymers.
 10. The substrate for a circuit structure of claim9, wherein said first polymer layer and said second polymer layer aremade of perfluorocyclobutane (PFCB).
 11. The substrate for a circuitstructure of claim 1, further comprising a first polymer layer appliedto said first polished side of said planar substrate mass, wherein saidfirst polymer layer has a refractive index that is greater than therefractive index of said polyimide substrate.
 12. The substrate for acircuit structure of claim 11 wherein said first polymer layer has athermo-optic coefficient that is substantially equal to a negative ofthe product of the index of refraction of said first polymer layer andthe coefficient of thermal expansion of said polyimide substrate. 13.The substrate for a circuit structure of claim 1, further comprising afirst metallic layer applied to said first polished side of said planarsubstrate mass, a piezoelectric layer applied over said first metalliclayer, a second metallic layer applied over said piezoelectric layer.14. The substrate for a circuit structure of claim 13, wherein saidpiezoelectric layer is PZT.
 15. The substrate for a circuit structure ofclaim 14, wherein said first metallic layer is platinum.
 16. A method ofpreparing a substrate for a circuit structure comprising: providing aplanar substrate mass of polyimide material having a first side and asecond side; and polishing said first side to a surface smoothnessbetween about 0.025 μinch and 100 μinch.
 17. The method of claim 16wherein the step of polishing comprises chemical mechanical polishing.18. The method of claim 16 further comprising: applying a first metalliclayer to said first polished side of said planar polyimide mass bysputtering.
 19. The method of claim 18, further comprising: shaping saidplanar polyimide substrate mass and said first metallic layer to formholes using at least one laser emitting in the ultraviolet spectralrange.
 20. A method of forming a groove having a precise depth and widthin a planar substrate mass of polyimide material comprising: providing aplanar substrate mass of polyimide material having a first side and asecond side; polishing said first side to a surface smoothness betweenabout 0.025 μinch and 100 μinch; and cutting said first side of saidplanar substrate mass using a pulsed laser, said pulsed laser outputtinga predetermined number of pulses to form a groove having a depth and awidth capable of receiving an optical fiber.